The present invention relates to a heat-resistant and oil-resistant laminate film and a process for preparing the same, and in detail, relates to the laminate film showing an improved heat-resistance and oil-resistance, having an adhesive layer made of a composition consisting essentially of a melt-extrudable adhesive resin and a radiation-sensitive compound and cross-linked by radiation treatment, and a process for preparing the laminate film.
More in detail, the present invention relates to a heat-resistant and oil-resistant laminate film comprising gas-barrier resin layer(s), thermoplastic resin layer(s) different from a gas-barrier resin and adhesive layer(s) disposed between the gas-barrier resin layer and the thermoplastic resin layer, produced by co-extruding a gas-barrier resin, a thermoplastic resin and an adhesive composition and irradiating the co-extruded laminate, the total number of the layers of the laminate film being not less than 3, and the adhesive layer comprising a cross-linked polymer and the adhesive composition consisting essentially of 100 parts by weight of a melt-extrudable adhesive resin and 0.1 to 50 parts by weight of a radiation-sensitive compound, and a process for preparing a coextruding a gas-barrier resin, a thermoplastic resin and an adhesive composition consisting essentially of 100 parts by weight of a melt-extrudable adhesive resin and 0.1 to 50 parts by weight of radiation-sensitive compound(s) in a configuration wherein an adhesive layer is disposed between a gas-barrier resin layer and a thermoplastic resin layer, thereby forming a multi-layered laminate consisting of not less than three layers, and irradiating the thus formed multi-layered laminate to cross-link the melt-extrudable adhesive resin.
In recent years, the demands for supplying heat-resistant and oil-resistant plastic packaging film have been increased, and particularly, those for supplying heat-resistant and oil-resistant packaging film for distribution and preservation for a long time period of foodstuffs containing much amount of animal- and vegetable oils and fat have been increased accompanying the qualitative improvement and diversification of the public's diet.
Not only the heat-resistance and oil-resistance, but also the properties as a gas-barrier, a heat-seal and a mechanical strength are demanded of such plastic packaging film for such foodstuffs and accordingly, the packaging film of a single layer cannot respond to such demands at all.
For those demands, laminate films produced by laminating several single layers respectively having various properties one after another, or produced by coextruding a plurality of layers respectively having various properties have been offered, however, since the adhesion between the layers showing mutually different properties is not necessarily favorable, there are many cases where it is necessary to use an adhesive layer between such layers.
Even in the case where a foodstuff containing animal- and/or vegetable oils and fats, for instance, raw meat, processed meat, cheese and the like, is packaged with a laminate film having conventional adhesive layer(s) and heat-resistant and oil-resistant outer layer(s) and the thus packaged foodstuff is subjected to sterilizing treatment at a high temperature, there are caused many defects as follows due to the poor resistance of the conventional adhesive layer(s) against heat and oil.
Namely, the peeling of the layer(s) occurs at the adhesive layer (s), the slipping occurs between the layers during the sterilizing treatment at a high temperature resulting in the remarkably damaged appearance of the package such as the increased degree of cloudiness caused by interfacial minute unevenness, and the minute wrinkles on the outermost surface occur.
Furthermore, at present when the conditions in sterilization have come to be severer than ever, a laminate film provided with an adhesive layer of improved resistance to heat and oil is eagerly demanded
Among the studies for obtaining an adhesive resin of improved resistance to heat and oil, there is a method of cross-linking the adhesive resin. The objective of the method is the improvement of the heat-resistance and oil-resistance of the adhesive layer by cross-linking the adhesive resin, thereby enlarging the domain of rubber-like elasticity thereof to the high temperature side to prevent the flow of the adhesive resin and to prevent the breakage thereof and the reduction of the strength and the peeling at break thereof at a high temperature due to the animal- and/or vegetable oils and fats which have been derived from the oily and/fatty foodstuff and have been taken up by the adhesive resin layer.
In general, as a method for cross-linking the adhesive resin layer, (i) the method of admixing an organic peroxide into the adhesive resin, (ii) the method of using an adhesive resin ionically cross-linkable and (iii) the method of cross-linking by silane-coupling have been proposed. However, the first method is not favorable because of the decomposition of the organic peroxide and the occurrence of bubbling during the coextrusion of the layers of laminate, the second method frequently causes the slipping between the layers of the laminate film by the breaking of the cross-linking at a high temperature as in the case of treatment in a retort resulting in the flow of the adhesive resin, and the silane-coupling has demerits of being complicated in operation, of requiring the treatment thereof with a hot water and steam, of causing contraction of the laminate film in the case where the laminate film is shrinkable and still requiring a long time of treatment thereof.
In particular, in Japanese patent application Laying Open No. 57-212282 (1982), a method of irradiating an electron beam onto a laminate film is disclosed by which the specified adhesive resin is subjected to cross-linking, thereby solidifying the adhesive layer. More in detail, according to the thus disclosed method, an adhesive agent containing a polymeric substance comprising a modified polymer of a specified acrylate ester having a glycidyl group is painted on the adhesive layer, and after laminating and sticking the other layer thereon or laminating thereof by extrusion coating thereon, the thus laminate film is irradiated by the electron beam.
However, the application of such a modified polymeric substance of a specified acrylate ester having glycidyl group as the adhesive layer is unsuitable for preparing the laminate film by coextrusion method because of the occurrence of gelation on melt-extruding of the adhesive resin layer resulting in the inability to continue the operation for a long time period and the formation of melt-flucture due to the large difference of melt-viscosities in the interface of the laminated layers.
As a result of the present inventors' studies for a favorable method of cross-linking the adhesive resin in the preparation of a laminate film being excellent in gas-barrier property, heat-resistance and oil-resistance, it has been found by the present inventors that a laminate film excellent in heat-resistance and oil-resistance can be obtained by coextruding the gas-barrier resin, the thermoplastic resin and the adhesive composition consisting essentially of a melt-extrudable resin and a radiation-sensitive compound and irradiating the extruded laminate, and the present inventors have attained the present invention.